1. Technical Field
The present invention relates to white toner.
2. Background Art
Full-color images using four colors of yellow, magenta, cyan, and black have been widely used in recent years although electrophotographic images are generally printed in black. In such a case, an image is formed on a white substrate such as paper by using these four color toners. However, good coloring image are not obtained on a colored substrate such as black paper or colored paper or a transparent substrate such as a transparent film by using only four color toners. For this reason, JP-2006-220694-A discloses a method of using white toner as the fifth color toner to make a white background image.
White toner is used to make a white background on a black or colored substrate such as paper, or used for a white background of a transparent substrate such as film. In this case, the white toner is required to have masking characteristics. The masking characteristics mean the ability to hide what exists below the background on which the white toner is fixed. In the case of white color, only a fixed white toner is used for white coloring and it is therefore necessary to scatter and reflect all incident light. If there is a little transmissive light, the obtained image is not vivid or clear.
To solve this problem, for example, JP-H01-105962-A and JP-2000-056514-A disclose improving masking characteristics.
Also, white toner has a specific problem about the fixability in attempts to achieve low temperature fixing by increasing the concentration of pigments to improve the masking characteristics.
Among efforts to solve this problem, JP-2010-008816-A discloses a method of improving the masking characteristics, the low temperature fixability, and the image strength by providing a wider range of selection of toner binder resins by a toner manufacturing method and selecting the surface treatment of titanium dioxide matching the toner manufacturing method. Although this is successful in some extent, the masking characteristics obtained by using a small amount of pigments, which gives rise to a problem of reducing the amount of toner attached.
Also, in general, it is not possible to form a white layer in one pass if the amount of toner attached is required to be more than in the case of typical process color. Accordingly, an attempt to solve this problem is increasing the particle diameter of the white toner. However, it is found that if the particle diameter of the toner is different from those of other color toners, a tandem machine using, in particular, an intermediate transfer element disturbs the other color toner image by transfer thereof.